Testing Coils

[csmith]

How do you test coils? I read the wiki but don't fully understand? What is the difference between Primary and Secondary readings and how do you test for each? Can this be done for coils by themselves and not in a distributor? IE NOS or TO? I have a Om meter but not really sure how to use it. (yes I'm a mechanic rookie, but learning ) I picked up a couple NOS coils a while back and want to test them, also have one in a T/O distributor wanting to test. Tx

[muttguru]

How do you test coils? I read the wiki but don't fully understand?

Clell,
Try this.
I have some information on coil testing, but note that even the US military didn't recommend doing this. The Mil always recommended substituting a doubtful coil with a known, good, one.

If you really want to check one out....the details are:-

1. Remove the six screws holding the distributor cap in place.
2. Disconnect the leads to the screw-terminals of the coil. These are the primary terminals.
3. Get an test-meter and set the meter to ohms (resistance);
4. Measure the resistance between the two primary terminals (the screw terminals);
5. Readings should show almost ZERO, just a few ohms at most (usually between 1.5 to 6 Ohms).
6. If you get a higher reading, then there is an internal break in the coil windings and coil is bad;
7. Reconnect the leads to the screw-terminals
8. Check the High Tension side by inserting an ordinary auto spark-plug lead in the HT outlet (the high-voltage central terminal of the coil) with the coil connected up and installed in the distributor, but with distributor cap removed.
7. Fit an ordinary spark-plug into the end of the spark-plug lead.
8. Hold the spark-plug against a good clean metal part of the engine block (paint-free). Make sure you use INSULATED pliers to hold the plug against the block or your hair will curl;
9. Get a friend to turn the ignition switch to "on" and crank the engine;
10. You should see a good spark jumping the plug-gap. If not, then the HT -side of the coil is failing.
11. Disconnect the leads to the screw-terminals. Now test the coil HT-side by connecting one lead of the ohmmeter to EITHER of the primary terminals (the screw-terminals of the coil), and connect the OTHER lead to the secondary terminal (the high-voltage central terminal of the coil) . The reading should be less than 20,000 ohms. Higher than this indicates an open circuit, a very low reading indicates a shorted internal winding.
A reading of around 14,000 ohms is normal.
12. If the coil readings are within the specs but vehicle won't start after running, or cuts out when running, test the coil immediately..... an internal-wire breakage is most likely when coil is warm or hot.

See "PS Magazine Issue 246 page 64 for resistance tests.

This is as good a checkout as is possible for the coil. The usual symptoms of a faulty coil are:-

1. Vehicle won't start, or -
2. Vehicle starts and runs fine, but won't re-start after switching engine off, or -
3. Vehicle comes to a complete stop and won't re-start but when an hour or so has elapsed, vehicle starts and runs fine for a while.

Remember - a coil which acts up, intermittently can test ok one minute, then let you down an hour later. If possible keep a known good coil as a spare.

Hope this helps.........

Kind regards as always....
Ken

[csmith]

Thanks Ken, so if just testing a coil out of a box or in a T/O distributor without a mutt nearby, to just get an initial feel for if the coil is good or not, one would preform items 4, 5, 6? And for the High Tension test the coil must be installed in a Dist in a Mutt?? Clell

[rickf]

Clell, to test the secondary you would go between either small terminal and the coil wire terminal. resistance should be around 15K ohms. Can be tested on the bench. Between the two small terminals should be 1.5 to 6 ohms.

[johnfal]

I have just bought a load of NOS coils, 16 all together and have cold tested them with an Ohm Meter. Out of that batch two failed. As Rick said you should get around 6.0 ohms +/- 0.5 ohms or thereabouts across the terminals. Between the Terminals and the High Voltage output you should get between 13.5K ohms to about 15.5K ohms or thereabouts. Of all the coils I have tested I have never had a good coil with a reading between the terminals of less than about 5.5 ohm. When testing the coils make sure the terminals are clean as oil, grease or grime can affect the reading. The two NOS I had that failed gave a reading of 6.0 ohm across the terminals but a reading of infinity (no connection) between the terminals and the HV connection.

One way of cold testing coils, electronic modules, capacitor, distributor cap, plug leads and plugs in a single operation without disturbing your jeep setup is to get an old distributor clamp it to a vice so that the shaft can turn. Install your coil and electronic module to be tested into the distributor (wired up correctly). Connect the positive lead from a 24V battery charger or battery to the positive connection on the distributor, connect the negative to the body of the distributor. connect a plug lead with a plug to any one of the plug connections on the cap, connect the plug to the negative of the charger or battery. Turn the shaft of the distributor so that it rotates clockwise when looking from the top and you should get a steady spark if all is working correctly. This is how one of my spare part outlets in Saigon tests distributors he sells, take about 5 minutes to set up.

Starting with a known good setup, you can just change any single part of it with any part that is suspect, also by moving the plug leads to different outlets on the caps you can test each connection separately, if you want to get clever fit four leads and plugs it will do the cap, lead and plug test in one go. You can also test your pace maker or heart using this method if you happen to have your fingers in the HV circuit when it fires

Warning you are playing with high voltages when testing live so stay alert and keep your sticky fingers away from the HV parts when spinning the distributor shaft.

[csmith]

Thanks guys, John is the high voltage output the deal in the center of the coil? Just looking at mine on the bench, not in the distributor. Can't access my tech manuals for the time being or I would look for a picture there with the info....

[johnfal]

Hi Clell, the two posts with the nuts are the input terminals, the recessed contact stud is the High Voltage terminal. If you are using a digital multimeter with auto ranging just set it to ohms, touch the terminals you wish to test between with the probes, polarity is not important. Hold the probes steady and wait until the reading stabilises. Depending on the type of multimeter the reading will be given as a complete number of ohms or with or with a symbol to indicate K ohmx or M ohms. No symbol indicates ohms.

If you are using an analogue meter select the range you wish to measure i.e. Ohms or K ohms, toch the probes together and make sure the reading on the meter is zero. If not adjust the set zero screw until the meter reads zero then take your reading. You need to do this everytime you change range. Rember the units you are reading and make a note on the coil body for future reference.

Hope this helps

John

[rickf]

I can give you a much simpler version of a live test if you are interested, no distributor needed. Will have to wait till lunchtime.

Rick

[johnfal]

I'm game Rick anything for an easier life. I have my own ideas for making a test rig without a distributor but until I test it I'll keep it under wraps

[rickf]

Parts needed.
Three jumper wires, 14-16 gauge, alligator clips on ends. Prefer two with black ends and one with red.
One military spark plug and wire.
One rubber band.
One battery, 12 volt should suffice.

The process.
For ease of connection I use aftermarket battery terminal ends and put them on the battery with no cables. This gives yuo something to clip onto.

Take your coil and tie the rubber band to the plug wire end opposite the plug about an inch from the end of the wire. Put the wire in the high tension hole and loop the rubber band around the coil. The rubber band is just to keep the wire from falling out. You can use anything you like here to catch the spark to ground, this is just the safest way I know of.

Attach the red wire from the positive battery terminal to the coil positive terminal. Attach one of the black wires to the grounding electrode of the spark plug. Attach the other black wire to the negative post of the coil and nothing else.

Now it is time to make sparks. Take the loose end of the black wire from the negative coil terminal and tap it against the battery negative terminal, You will only hold it on there a second and remove. As you remove you will get a spark.

Now if you really want to get fancy you can put a momentary push button in the line and hook the black line to the battery and use the button instead of tapping. I like the KISS theory myself.

The one nice thing about the distributor setup with a motor is that you could test for heat related failure over time with a heat gun on the distributor. Just watch out you do not get it too hot or you can cause your own destruction. A 12 volt battery will not power the electronic unit nor will it build heat in the coil. I do know a guy who has been running a 151 setup in a swamp buggy for many years on 12 volts. He told me it runs great, especially since he got the timing straightened out.

Rick

[csmith]

Got a battery for my meter today and did a bench test, one coil 6.3/14.3om, one 6.1/14.5om so I'm good. These are the real deal NOS US made coils not the chrome China ones so its good to know I have good spares in the inventory. Thanks to everyone for the help, one more lesson learned

[Lou]

Youtube video of testing coil and distributor. https://www.youtube.com/watch?v=EKhQ5sF ... Yo&index=2